A typical gas turbine engine has a compressor section, a combustion section, and a turbine section. The gas turbine engine includes an annular flowpath for conducting working fluid sequentially through the compressor section, the combustion section, and the turbine section. The compression section adds energy in form of momentum to the working fluid. The combustion section mixes fuel with the compressed working fluid and combusts the mixture. The products of combustion are expanded through the turbine section. The turbine section includes an array of airfoil shaped blades attached to rotating disks. The interaction of the working fluid and the turbine blades transfers energy to the rotating disks. The rotating disks are connected to the compressor section by a shaft. In this way, a portion of the energy removed from the expanding working fluid is used to compress incoming working fluid in the compressor section.
The output of the gas turbine engine is dependent in part upon the energy added to the fluid in the combustion section. The combustion section adds energy in the form of heat to the working fluid. The amount of heat added to the working fluid is limited by the temperature characteristics of the turbine section components. The turbine blades, disks and other turbine structure have material temperature characteristics which limit the temperature of the working fluid exiting the combustion section.
One particular area of concern in gas turbine engines is the blade attachment mechanism of the rotating disk. Typically, the disk has a plurality of axially oriented dove-tail or fir-tree shaped slots. The plurality of blades have root portions which are shaped to accommodate the slot to provide a retaining mechanism against radially outwardly directed rotational forces. The high rotational speeds of the disk causes the blade attachment region to be an area of very high stress in the disk. The allowable stress of the disk material for either static loading or fatigue loading, decreases as the temperature of the disk increases.
The disk attachment stress rupture life may be extended by either reducing the stress in the disk or by reducing the temperature of the highly stressed region of the disk. Reducing the stress in the disk may be accomplished by reducing the size and weight of the blades attached to the disk. In most situations, however, the size and design of the blades has been optimized for efficient performance of the gas turbine engine. Therefore, reducing the stress by altering the size and weight of the blades may not be a practical option. Reducing the temperature of the blade attachment region of the disk has been accomplished with some measure of success in the prior art. In U.S. Pat. No. 3,733,146, issued to Smith and Voyer, entitled "Rotating Seal For A Gas Turbine Engine", a cover plate for the disk attachment region was disclosed. The cover plate provided a aerodynamically smooth flow surface to reduce windage losses in the blade attachment region of the disk. In U.S. Pat. No. 4,659,285, issued to Kalogeros and Chaplin, entitled "Turbine Cover Seal Assembly", an improved cover plate for the blade attachment region of the disk was disclosed. This cover plate provided both a windage cover and insulated the disk rim from the working fluid.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop methods and apparatus for minimizing the temperature of the blade attachment region of rotating disks.